1. Field of the Invention
The subject invention generally pertains to electronic power conversion circuits, and more specifically to high frequency, switched mode power electronic converter circuits.
2. Description of Related Art
There are some power conversion circuits which accomplish higher efficiencies by implementing a mechanism that accomplishes switching at zero voltage. Power loss in a switch is the product of the voltage applied across the switch and the current flowing through the switch. In a switching power converter, when the switch is in the on state, the voltage across the switch is zero, so the power loss is zero. When the switch is in the off state, the power loss is zero, because the current through the switch is zero. During the transition from on to off, and vice versa, power losses can occur, if there is no mechanism to switch at zero voltage or zero current. During the switching transitions, energy losses will occur if there is simultaneously (1) non-zero voltage applied across the switch and (2) non-zero current flowing through the switch. The energy lost in each switching transition is equal to the time integral of the product of switch voltage and switch current. The power losses associated with the switching transitions will be the product of the energy lost per transition and the switching frequency. The power losses that occur because of these transitions are referred to as switching losses by those people who are skilled in the art of switching power converter design. In zero voltage switching converters the zero voltage turn off transition is accomplished by turning off a switch in parallel with a capacitor and a diode when the capacitor's voltage is zero. The capacitor maintains the applied voltage at zero across the switch as the current through the switch falls to zero. In the zero voltage transition the current in the switch is transferred to the parallel capacitor as the switch turns off.
The zero voltage turn on transition is accomplished by discharging the parallel capacitor using the energy stored in a magnetic circuit element, such as an inductor or transformer, and turning on the switch after the parallel diode has begun to conduct. During the turn on transition the voltage across the switch is held at zero, clamped by the parallel diode. The various zero voltage switching (ZVS) techniques differ in the control and modulation schemes used to accomplish regulation, in the energy storage mechanisms used to accomplish the zero voltage turn on transition, and in a few cases on some unique switch timing mechanisms.
A DC transformer is a circuit that transforms voltages from an input DC voltage to an output DC voltage. Typically the DC transformer also provides galvanic isolation between the input circuits and the output circuits. The circuit typically contains a switch or a set of switches that transform the input DC voltage to an AC voltage which is applied to a transformer primary winding or a set of primary transformer windings. The secondary windings of the transformer will have AC signals that are analogous to the AC signals that appear on the primary windings, but the secondary signals will be scaled by the transformer turns ratio. The signals that appear at the secondary windings are rectified to form a DC voltage. Common DC transformers are well known to those skilled in the art of power conversion. A discussion of DC transformers appears in the book by Severns and Bloom entitled "Modern DC-To-DC Switchmode Power Converter Circuits". DC transformers are commonly used in combination with the common buck, boost, and buck boost converters to form complete power converters. Typically a buck or a boost converter is used as a pre-regulator to the DC transformer to form the converter system, but the buck, boost, or buck boost converter may also be used as a post regulator. The DC transformer operates at approximately 100% duty cycle but provides no duty cycle variability so the pre or post regulator is needed to provide the necessary regulation.
One example of a DC transformer is shown in FIG. 1. The two switches on the input side form an AC signal which is applied to the primary winding of an ideal transformer. The two switches are operated alternately, each at approximately 50% duty cycle. The two secondary windings provide scaled versions of the signals that appear on the primary winding. The two secondary switches are operated alternately, each at 50% duty cycle. The action of the two secondary switches is to rectify the secondary signals forming a DC voltage at the output capacitor. One problem with the FIG. 1 circuit is that the two secondary windings must both be tightly coupled to the primary winding, but only one secondary winding is active at any time. The inactive secondary winding will contribute eddy currents and AC winding losses and the two secondary winding construction will contain a high amount of leakage inductance because leakage flux will exist in the space occupied by the inactive winding. The combination of high leakage inductance and high AC winding losses result in a DC transformer which is less than optimal for high frequency operation.
Another example of a DC transformer is shown in FIG. 2. The FIG. 2 circuit contains two distinct and separate transformers. The two transformers are operated alternately at approximately 50% duty cycle. In each of the two transformers there is a single primary winding and a single secondary winding which can be tightly coupled to obtain both low leakage inductance and low AC winding losses. There is one shortcoming of the FIG. 2 circuit, the inactive transformer will ring with the circuit capacitive parasitics during the inactive period of operation since there is no clamping mechanism for any transformer winding. During the active period of the transformer, energy will build in the transformer core due to the magnetizing current. When the transformer becomes inactive by turning off the primary switch connected to that transformer the energy stored in the core will ring with the switches parasitic capacitances and the intra-winding capacitances of the transformer. This ringing creates EMI and the need for a snubber to damp the ringing and/or a clamp circuit to protect the switches from over voltage.
The FIG. 3 circuit is a modification of the FIG. 2 circuit that contains a tertiary winding in each transformer and a rectifier. The tertiary winding and the rectifier form a clamp which can reduce the ringing and provide relatively square wave forms which may be suitable for secondary synchronous rectifier self drive. The problem with the tertiary winding is that it adds AC winding losses when the tertiary winding is inactive and it adds to the leakage inductance. It also adds DC winding losses because the window area occupied by the tertiary winding reduces the window area available to the primary and secondary windings thus increasing the winding resistance of either or both of the other windings. The effectiveness of the clamp depends on how tightly the tertiary winding is coupled to the winding that it clamps. In the FIG. 3 circuit the primary winding must be tightly coupled to both the secondary winding for low leakage inductance and high efficiency and it must also be tightly coupled to the tertiary winding in order to accomplish an effective clamp. To some extent these two requirements are mutually exclusive. In general both the coupling coefficient and efficiency are compromised by the addition of a special clamp winding.